10 August 2009
The invention: Double sheets of glass separated by a thin layer of plastic sandwiched between them. The people behind the invention: Edouard Benedictus (1879-1930), a French artist Katherine Burr Blodgett (1898-1979), an American physicist The Quest for Unbreakable Glass People have been fascinated for centuries by the delicate transparency of glass and the glitter of crystals. They have also been frustrated by the brittleness and fragility of glass. When glass breaks, it forms sharp pieces that can cut people severely. During the 1800’s and early 1900’s, a number of people demonstrated ways to make “unbreakable” glass. In 1855 in England, the first “unbreakable” glass panes were made by embedding thin wires in the glass. The embedded wire grid held the glass together when it was struck or subjected to the intense heat of a fire.Wire glass is still used in windows that must be fire resistant. The concept of embedding the wire within a glass sheet so that the glass would not shatter was a predecessor of the concept of laminated glass. A series of inventors in Europe and the United States worked on the idea of using a durable, transparent inner layer of plastic between two sheets of glass to prevent the glass from shattering when it was dropped or struck by an impact. In 1899, Charles E.Wade of Scranton, Pennsylvania, obtained a patent for a kind of glass that had a sheet or netting of mica fused within it to bind it. In 1902, Earnest E. G. Street of Paris, France, proposed coating glass battery jars with pyroxylin plastic (celluloid) so that they would hold together if they cracked. In Swindon, England, in 1905, John Crewe Wood applied for a patent for a material that would prevent automobile windshields from shattering and injuring people when they broke. He proposed cementing a sheet of material such as celluloid between two sheets of glass. When the window was broken, the inner material would hold the glass splinters together so that they would not cut anyone.Remembering a Fortuitous Fall In his patent application, Edouard Benedictus described himself as an artist and painter. He was also a poet, musician, and philosopher who was descended from the philosopher Baruch Benedictus Spinoza; he seemed an unlikely contributor to the progress of glass manufacture. In 1903, Benedictus was cleaning his laboratory when he dropped a glass bottle that held a nitrocellulose solution. The solvents, which had evaporated during the years that the bottle had sat on a shelf, had left a strong celluloid coating on the glass. When Benedictus picked up the bottle, he was surprised to see that it had not shattered: It was starred, but all the glass fragments had been held together by the internal celluloid coating. He looked at the bottle closely, labeled it with the date (November, 1903) and the height from which it had fallen, and put it back on the shelf. One day some years later (the date is uncertain), Benedictus became aware of vehicular collisions in which two young women received serious lacerations from broken glass. He wrote a poetic account of a daydream he had while he was thinking intently about the two women. He described a vision in which the faintly illuminated bottle that had fallen some years before but had not shattered appeared to float down to him from the shelf. He got up, went into his laboratory, and began to work on an idea that originated with his thoughts of the bottle that would not splinter. Benedictus found the old bottle and devised a series of experiments that he carried out until the next evening. By the time he had finished, he had made the first sheet of Triplex glass, for which he applied for a patent in 1909. He also founded the Société du Verre Triplex (The Triplex Glass Society) in that year. In 1912, the Triplex Safety Glass Company was established in England. The company sold its products for military equipment in World War I, which began two years later. Triplex glass was the predecessor of laminated glass. Laminated glass is composed of two or more sheets of glass with a thin layer of plastic (usually polyvinyl butyral, although Benedictus used pyroxylin) laminated between the glass sheets using pressure and heat. The plastic layer will yield rather than rupture when subjected to loads and stresses. This prevents the glass from shattering into sharp pieces. Because of this property, laminated glass is also known as “safety glass.” Impact Even after the protective value of laminated glass was known,the product was not widely used for some years. There were a number of technical difficulties that had to be solved, such as the discoloring of the plastic layer when it was exposed to sunlight; the relatively high cost; and the cloudiness of the plastic layer, which obscured vision—especially at night. Nevertheless, the expanding automobile industry and the corresponding increase in the number of accidents provided the impetus for improving the qualities and manufacturing processes of laminated glass. In the early part of the century, almost two-thirds of all injuries suffered in automobile accidents involved broken glass. Laminated glass is used in many applications in which safety is important. It is typically used in all windows in cars, trucks, ships, and aircraft. Thick sheets of bullet-resistant laminated glass are used in banks, jewelry displays, and military installations. Thinner sheets of laminated glass are used as security glass in museums, libraries, and other areas where resistance to break-in attempts is needed. Many buildings have large ceiling skylights that are made of laminated glass; if the glass is damaged, it will not shatter, fall, and hurt people below. Laminated glass is used in airports, hotels, and apartments in noisy areas and in recording studios to reduce the amount of noise that is transmitted. It is also used in safety goggles and in viewing ports at industrial plants and test chambers. Edouard Benedictus’s recollection of the bottle that fell but did not shatter has thus helped make many situations in which glass is used safer for everyone.